The present invention relates to a sensor such as an electrostatic capacitance type pressure sensor for detecting the pressure of a medium to be measured or acceleration sensor for measuring an acceleration, and a method of manufacturing the same and, more particularly, to an improvement of electrode extraction structures arranged to face each other.
In an electrostatic capacitance type pressure sensor, a plate-like stationary electrode and movable electrode are arranged parallel and close to each other so as to face each other within the cavity of a sensor main body. A change in capacitance between the two electrodes along with displacement of a diaphragm is detected to measure the pressure of a medium to be measured. Various pressure sensors have conventionally been proposed, as disclosed in Japanese Patent Laid-Open No. 6-265428 (to be referred to as a prior art). The electrode extraction structure in such pressure sensor is generally constituted by bonding a connection pin inserted through a cavity from an electrode extraction hole to an electrode with a bonding agent such as solder or paste (prepared by kneading a metal powder with a binder)
FIG. 12 is a sectional view showing an electrostatic capacitance type pressure sensor described in the prior art. FIG. 13 is a sectional view taken along the line D-Dxe2x80x2 in FIG. 12. FIG. 14 is a sectional view taken along the line E-Exe2x80x2 in FIG. 12.
As shown in FIG. 12, a sensor main body 202 is formed by first and second sapphire substrates 202A and 202B directly bonded to each other. A stationary electrode 201 and movable electrode 203 are arranged parallel to face each other in the sensor main body 202. This structure constitutes the electrostatic capacitance type sensor.
The first substrate 202A is thicker than the second substrate 202B. Three electrode extraction holes 204 and one atmospheric pressure inlet hole 205 are formed through the first substrate 202A in the direction of thickness. These holes 204 and 205 allow a cavity 207 formed in the sensor main body 202 to communicate with the outside.
A recess 208 is formed at the center of the inner surface of the second substrate 202B that faces the first substrate 202A. A space defined by the recess 208 and the inner surface of the first substrate 202A forms the cavity 207. The central portion of the second substrate 202B is made thin by the recess 208 to form a diaphragm 209.
The second substrate 202B is directly bonded to the first substrate 202A through a thick outer peripheral portion 210. The first and second substrates 202A and 202B are made of the same material, do not sandwich any interposition on their bonded surface, and thus are almost free from any residual stress of bonding. The first and second substrates 202A and 202B can be used without causing any change over time which deforms the diaphragm 209, and can provide stable sensor characteristics.
The movable electrode 203 is made up of a sensing electrode 203A having pressure sensitivity, and reference electrode 203B having almost no pressure sensitivity. An output difference between the two electrodes 203A and 202B can be detected to cancel the influence of a temperature change and environmental change.
The three electrode extraction holes 204 formed in the first substrate 202A, i.e., holes 204a, 204b, and 204c shown in FIG. 14 correspond to the stationary electrode 201, sensing electrode 203A, and reference electrode 203B, respectively. The electrode extraction hole 204a is formed at a position where the hole 204a extends through an electrode extraction portion 214 formed on the stationary electrode 201. The electrode extraction holes 204b and 204c are respectively formed at positions corresponding to electrode extraction portions 215A and 215B formed on the sensing electrode 203A and reference electrode 203B.
Electrode extraction will be described. FIG. 15 is an enlarged sectional view taken along the line F-Fxe2x80x2 in FIG. 13. FIG. 16 is an enlarged sectional view taken along the line G-Gxe2x80x2 in FIG. 13.
After the first substrate 202A is directly bonded to a wafer serving as a substrate material of the second substrate 202B, the wafer is divided into chips by dicing. Connection pins 211 each having a lower end coated with a solder portion (or conductive paste) 212 are sequentially inserted (in practice, press-inserted) into the electrode extraction holes 204b and 204c for the movable electrode 203, and brought into contact with the electrode extraction portions 215A and 215B of the sensing electrode 203A and reference electrode 203B, respectively. The structure is heated in this state to temporarily fuse the solder portions 212, and then the solder portions 212 are cooled and solidify. This electrically connects the sensing electrode 203A and reference electrode 203B to the connection pins 211, as shown in FIG. 15.
Further, as shown in FIG. 16, a connection pin 211 having a lower end coated with a conductive paste 213 (or solder portion) is press-inserted into the electrode extraction hole 204a for the stationary electrode 201. The stationary electrode 201 and connection pin 211 are electrically connected through the conductive paste 213.
[Problem to be Solved by the Invention]
As described above, in the conventional electrostatic capacitance type pressure sensor, the electrode 201 or 203 is mechanically, electrically connected to the connection pin 211 using the solder portion 212 or conductive paste 213. This poses the following problems.
More specifically, when the movable electrode 203 is connected to the connection pin 211 using the solder portion 212 as a bonding agent, a sufficient bonding strength cannot be obtained with low wettability of the movable electrode 203. With high wettability, the solder portion 212 flows from the electrode extraction portion 215A or 215B to short-circuit the stationary electrode 201 and movable electrode 203.
When the conductive paste 213 is used as a bonding agent, a decrease in bonding strength and connection errors occur in an excessively small amount of conductive paste 213, similar to the solder portion 212. In an excessively large amount of conductive paste 213, the conductive paste 213 contacts the stationary electrode 201 to short-circuit the stationary electrode 201 and movable electrode 203.
On the other hand, when the stationary electrode 201 is connected to the connection pin 211, the stationary electrode 201 is not formed on the abutment surface of the connection pin 211, as shown in FIG. 16. In other words, the stationary electrode 201 is formed on the inner surface of the sensor main body 202 in which the electrode extraction hole 204a is formed. For this reason, the solder portion is difficult to electrically connect the stationary electrode 201.
The stationary electrode 201 may be somehow electrically connected using the conductive paste 213. However, similar to the movable electrode 203, the stationary electrode 201 may short-circuit with the movable electrode 203 or fail in connection depending on the amount of conductive paste 213.
[Means of Solution to the Problem]
The present invention has been made to overcome the conventional drawbacks, and has as its object to realize a sensor having a high bonding strength between an electrode and a connection member such as a connection pin.
It is another object of the present invention to realize a sensor capable of preventing electrodes from short-circuiting with each other.
It is still another object of the present invention to realize a sensor capable of reliably electrically connecting the connection member and electrode.
It is still another object of the present invention to realize a sensor high in yield and excellent in mass production.
To achieve the above objects, a sensor according to the present invention is characterized by comprising a sensor main body (2) in which a cavity (7) which communicates with outside through a plurality of electrode extraction holes (4a, 4b, 4c, 31, 104a, 104b, 104c, 131) is formed, a pair of electrodes (1, 3) arranged in the cavity to face each other, a plurality of connection members (11, 12, 111) which are inserted in the cavity through the electrode extraction holes and are electrically connected to corresponding electrodes, and pads (24, 25) which are formed in the cavity at positions where the pads face the electrode extraction holes, are bonded to corresponding connection members, and are made of a conductive material, wherein wettability of each connection member for a pad surface is higher than wettability of the connection member for an electrode surface. Since the connection member has high wettability for the pad surface, part of the fused connection member spreads on the entire pad surface to firmly bond the connection member to the pad. As a result, the connection member can be reliably electrically connected to the electrode. To the contrary, the connection member has low wettability for the electrode surface. Even if part of the fused connection member overflows from the pad, it hardly flows along the electrode surface. For this reason, the connection member does not contact both a pair of electrodes, and does not short-circuit them.
As an arrangement of the sensor, the pad (24) is formed on a surface of the electrode (3, 3A, 3B, 15A, 15B) present at a position where the electrode faces the electrode extraction hole (4a, 4b, 4c, 31), and the electrode is connected to a corresponding connection member (11, 12) through the pad. With this structure, the electrode can be extracted through the connection member in a direction different from the electrode direction.
In this case, an arrangement of the connection member comprises a connection pin (11) inserted through the cavity from the electrode extraction hole, and a bonding agent (12) for bonding the connection pin to the pad. Since the connection member is formed by bonding the connection pin to the pad with the bonding agent, the connection member can be reliably electrically connected to the electrode.
When the bonding agent is made of Snxe2x80x94Ag, the sensor may be constituted using Pt as the material of the electrode surface and Au as the material of the pad surface. This is because the wettability of Snxe2x80x94Ag is high for Au and low for Pt.
As another arrangement of the sensor, the pad (25) is formed at a position where the pad faces the electrode extraction hole, on a surface (21b) opposite to the electrode extraction hole side among inner surfaces (21a, 21b) of the sensor main body forming the cavity, and the electrode (1) formed on the electrode extraction hole side contacts a corresponding connection member (11, 12). With this structure, the electrode can be extracted through the connection member in the same direction as the electrode direction.
In this case, another arrangement of the connection member comprises a connection pin (11) inserted into the cavity from the electrode extraction hole, and a bonding agent (12) for bonding the connection pin to the pad and the electrode formed on the electrode extraction hole side. Since the pad having high in wettability for the bonding agent is arranged at an opposite position, the bonding agent can reach the electrode surface through the connection pin to reliably electrically connect the electrode and connection pin. This reduces connection errors.
When the bonding agent is made of Snxe2x80x94Ag, the sensor may be constituted using Pt as the material of the electrode surface and Au as the material of the pad surface.
Still another example of the connection member in the sensor is made of solder for filling a space above the pad in the cavity and the electrode extraction hole (104a, 104b, 104c, 131). Since the electrode extraction hole is filled with the solder which is used as the connection member, the connection member can be easily formed.
When the solder is made of Snxe2x80x94Ag, the sensor may be constituted using Pt as the material of the electrode surface and Au as the material of the pad surface.
In this case, the electrode extraction hole may be tapered to decrease an inner diameter from an outer surface to inner surface of the sensor main body. This tapered shape can smoothly flow the fused solder.
In this case, at least part of an inner surface of the electrode extraction hole may be covered with a material having high wettability for the solder. This can easily flow the fused solder through the electrode extraction hole.
A sensor manufacturing method is characterized by comprising the steps of preparing a member in which a cavity (7) communicating with outside through a plurality of electrode extraction holes (104a, 104b, 104c, 131) is formed in a sensor main body (2), a pair of electrodes (1, 3) are arranged in the cavity to face each other, and a plurality of pads (24, 25) made of a conductive material higher in wettability for solder (111) than for the electrode are formed in the cavity at positions where the pads face the electrode extraction holes, flowing the fused solder from the electrode extraction holes onto the pads in the cavity to fill a space above the pads in the cavity and the electrode extraction holes with the fused solder, and cooling the fused solder to extract the electrodes in the cavity through the solder filled in the electrode extraction holes. This can form a sensor using the solder filling the electrode extraction hole as a connection member.
In this case, the sensor manufacturing method may further comprise disposing a solder lump (111A) having a predetermined size at an inlet of each electrode extraction hole on an outer surface of the sensor main body, heating and fusing the solder lump, and flowing the solder lump from the electrode extraction hole.
The sensor manufacturing method further comprises attaching a jig (127) around the inlet of the electrode extraction hole, and disposing the solder lump. The solder lump can be reliably disposed at a desired position.
The sensor manufacturing method further comprises increasing an external pressure of the sensor main body to be higher than an internal pressure of the cavity, and heating and fusing the solder. The fused solder can be forcibly flowed onto the pad.
In this manufacturing method, when the pad (24) is formed on a surface of the electrode (3A, 3B, 15A, 15B) present at a position where the electrode faces the electrode extraction hole, the solder is bonded to the pad. The electrode on an opposite side to the electrode extraction hole side can be extracted through the connection member made of the solder.
To the contrary, when the pad (25) is formed at a position where the pad faces the electrode extraction hole, on a surface (21b) opposite to the electrode extraction hole side among inner surfaces (21a, 21b) of the sensor main body forming the cavity, the solder is brought into contact with the electrode (1) formed on the electrode extraction hole side. Hence, the electrode on the electrode extraction hole side can be extracted through the connection member made of the solder.
In this manufacturing method, the electrode extraction hole can be formed using laser machining to taper the electrode extraction hole.